Operating assemblies of motor vehicles and electrical household appliances often have control panels with display and switching elements. In modern control panels, switching elements are often designed as non-contact capacitive touch sensors having conductive surfaces situated on the rear side of a flat control panel.
DE 10 2006 039 133 A1 describes an operating device having a capacitive sensor button(s). The sensor button is an electrically conductive sensor element having a sensor surface. The sensor surface rests against the rear side of an electrically insulating cover plate. A circuit board is situated at a distance from the cover plate. The sensor element extends from the cover plate to the circuit board. A light box is between the cover plate and the circuit board. Via the lightbox the sensor element is fixed in a predefined position on the rear side of the cover plate, in parallel and perpendicular to the cover plate.
This operating device is a part of a household appliance such as a washing machine or dishwasher. In such household appliances, it is generally not a problem or even advisable to manufacture an operating device with an integrated electronics system and to mechanically connect the circuit board bearing the electronics system to a control panel.
More problematic are devices in which an electronic unit is pre-installed in a module housing and sensors and/or display elements are to be mounted on a separate control panel to, for example, the dashboard of a motor vehicle.
In principle, flexible circuit carriers such as conductive foils or films may be provided in order to mount sensors and/or display elements on the rear side of a control panel. In known applications, conductive foils bearing capacitive sensor surfaces as flat strip conductor sections are glued directly behind a control panel or attached to the control panel by extrusion coating. The sensor surfaces and/or display elements must be contacted and connected to an associated evaluation electronics system. This often results in difficulties in installation, especially in limited installation space. In addition, only after final installation is it possible to check or examine the overall functionality. It is also disadvantageous that such an arrangement is difficult to uninstall or dismantle if required.
Sensor surfaces on a conductive foil may, in principle, be fastened to a control panel by mechanical pressing. For example, the conductive foil may be mechanically pressed against the control panel via an elastically attached or resiliently mounted housing. Complicated fixing by gluing or extrusion coating is therefore dispensed with.
In regards to mechanically pressing conductive foils to control panels, it is problematic that control panels, and in particular the rear sides of dashboards, often have curvatures, uneven areas, bumps, bulges, etc. As a result of a control panel having such surface distortions, it is difficult to uniformly press the functional section of a flexible conductive foil, especially a relatively wide flexible conductive foil, onto the control panel. In particular, when the conductive foil having the sensor surfaces does not lie close against the control panel the function of the capacitive sensors may be impaired. The sensitivity and functional reliability of capacitive sensors may be significantly reduced by air inclusions between the sensor elements and the control panel on account of the very low relative permittivity of air compared to plastic.
Also, for multi-part or flat display elements situated on the rear side of a transparent or semi-transparent control panel, it is advantageous when the display elements maintain a small, uniform distance from the rear side of the control panel in order to avoid interfering uneven brightness distributions or regionally unclear displays.